Photographic lens



SEARCH ROOM WTWLE (No Model.)

v A. G. CLARK.

PHOTOGRAPHIG- LENS.

No; 399.499. Patented Mar. 12. 1889.

UNITED STATES PATENT ALVAN G. CLARK, OF CAMBRIDGEPORT. MASSACHUSETTS.

PHOTOGRIAPHIC LENS.

SPECIFICATION forming part of Letters Patent No. 899,499, dated March 12, 1889.

Application filed October 4, 1888. Serial No. 287,202. No model.)

T 0 all whom it may concern.-

Be it known that I, ALVAN Cr. CLARK, a citizen of the United States, residing at Cambridgeport, in the county of Middlesex, and the State of Massachusetts, have invented certain new and useful Improvements in Photographic Tubes or Lenses, of which the folrapidity, depth of focus, a flat field, and the lowing is a specification.

My invention relates to various new and useful 1m1 )rovements in the construction and I proximated heretofore, except by the use of arrangementof lenses in photographic tubes, of which the following is a true description, reference being had to the accompanying drawing, which shows a section of a tube and lenses, in which X represents the front combination," so called, and'Y the back combibation.

produce a superior tube at a small expense, owing to the comparative facility with which lenses may be ground on spherical lines.

By the practice of my invention or discovery many of the greater ditficulties attendant on the production of photographic tubes are obviated. The requirements of a tube are absence of chromatic aberration or spherical distortion. Such a tube has not been aplenses ground to complicated curves, which render them very expensive, and results in a conside-able variation of excellency in the completed article.

The drawing, which is made a part of this :specification, shows the arrangement of a In the practice of my invention or discovcry each of the lenses used in the construction of the tube is ground to spherical curves,

the front combination, X, being similar in construction and arrangement to the back 1 combination, Y, with the crown-glass lenses 1 outermost in each of said combinations, while i the inside lenses, B and 0, used to achromatize the front and back combination, respectively, are made from flint-glass.

In the manufacture of photographic tubes 3 the lenses of the back combination have been separated by a ring of blackened metal, which is invariably the practice in portrait-tubes; but those of thefront combination are accurately ground to fit each other and cemented i firmly together.

Itis in reference to the front combination that my invention differs more materially from any photographic tubes heretofore made Z or known, inasmuch as the lenses of said? front combination are separated from each i 1: other to a greater or less degree, according to 1 Bv the kind of work required of the tube. this arrangement I am enabled to secure a large image in which the focus will be of nearly the same excellence throughout, or

that quality in photographs technically called flatness in the field." Moreover, I am enabled to place the crown-glass outermost,which,

being the harder glass, is less liable to be- 5 come scratched by use; and at he ame time tube of twelve-inch focus, approximately, insection, in which the lenses are ground to thefollowing radii: a and a, 2.05; b and b, 6.3; c and c, 3.3; (Z and d, 2.4-. Other measurements may often be used to ad\ 'antage, but these are deemed very efficient.

Having thus described myinvention, I desire to secure by Letters Patent the following claims:

1. In a photographic tube, the combination of a concavo-convex crown-glass lens, D, ground on spherical lines, and a concavo-convex flint-glass lens, C, ground on spherical lines, with a front combination consisting of two similar lenses separated from each other by a ring of blackened metal or substitute i therefor, substantially as described and set forth.

'2. In a photographic tube, a front combination, X, composed of two lenses having,

their respective surfaces both ground to curves which are practically spherical and separated from each other, substantially as described.

ALVA)? G. CLARK. Witnesses:

, HORACE W. WHIT EY, n-Infinite a, i 'nrrxav. 

